RCryptorv20HideEPVaska

Hunt Hypothesis

This detection identifies the presence of the RCryptor v20 ransomware variant utilizing endpoint hiding and VASKA obfuscation techniques to evade standard security controls. Proactive hunting for this signature in Azure Sentinel is essential to uncover early-stage lateral movement or initial infection vectors before encryption activities trigger broader alerts, given the rule’s low severity may otherwise result in delayed response.

YARA Rule

rule RCryptorv20HideEPVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { F7 D1 83 F1 FF 6A 00 F7 D1 83 F1 FF 81 04 24 DC 20 ?? 00 F7 D1 83 F1 FF E8 00 00 00 00 F7 D1 83 F1 FF C3 }

condition:
		$a0 at pe.entry_point
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the RCryptorv20HideEPVaska detection rule, including suggested filters and exclusions:

• Enterprise Antivirus Real-Time Scanning of Encrypted Archives

  • Scenario: The endpoint protection engine (e.g., Microsoft Defender or CrowdStrike) performs real-time scanning on large .7z or .rar archives containing encrypted data. The YARA rule may misinterpret the high entropy and specific header signatures of these compressed containers as the RCryptorv20 malware behavior, triggering an alert when a user opens a legitimate backup file.
  • Filter/Exclusion: Add a process exclusion for the AV engine’s core service (e.g., MsMpEng.exe or csagent.exe) when it is interacting with files in standard backup directories like C:\Backups\ or D:\Archive\.

• Scheduled Database Backup Jobs Using Compression Tools

  • Scenario: A nightly scheduled job using tools like 7-Zip Command Line (7z.exe) or WinZip runs to compress and encrypt SQL database backups. The rule detects the rapid file creation, high entropy output, and specific API calls used by these compression utilities as suspicious encryption activity typical of RCryptorv20.
  • Filter/Exclusion: Exclude processes running under the context of the backup service account (e.g., NT SERVICE\SQLAgent) specifically for paths matching C:\Program Files\PostgreSQL\ or D:\DB_Backups\.

• Software Deployment via Configuration Management Tools

  • Scenario: During a patch deployment cycle, tools like SCCM (System Center Configuration Manager) or Ansible execute scripts to install encrypted software packages. The installation process involves unpacking encrypted payloads and writing them to the disk, which mimics the “hiding” and encryption behaviors targeted